LM25145: LM25145RGYR FB regulation voltage level

Hey Dinesh,

Looks like your resistor divider works in theory with ideal values to get you to 3.3V out, but seems as if the values may be a bit off.

Do you know what the tolerances on the resistors you are using are? If possible it would be good to take the resistors off the board and double check the values to see if they are within tolerance. You need the devices to be pretty accurate to ensure your VOUT is within your desired range. 

The FB voltage and VOUT voltage is about ~3.75% higher than the desired 3.3 VOUT, so you could try by using a slightly smaller upper resistor value or slightly larger bottom resistor value. Generally we recommend using larger resistors to pull less current, for 3.3VOUT a 100k top resistor and a 32k bottom resistor would work well.

Thanks,

Dino Maslic

Hi Dino, Thank you for the quick response.!

Feedback resistor tolerance is +/- 1% in the design.
I will try measuring the feedback resistors by taking out from PCB once I have access to the hardware.

But I still have query on FB pin regulation.
Irrespective of resistor tolerance, VR should regulate FB pin at 0.8V +/-1%, isn't it?

If FB pin is regulated at 0.8V and if we have high or lower VOUT, then I believe it could be due to incorrect feedback resistor value. So I believe, 3.42V seen in our case seems different than incorrect feedback resistor value.

My concern is why VR not able to regulate FB pin at 0.8V, +/-1%?
Do other VR pins such as COMP pin or RT pin can influence VOUT value?

RT pin is connected to 18.2K pulldown resistor, which translates to 550KHz switching frequency in our design. 

Thanks, Dinesh

Hi Dinesh,

Understand your concern with the FB pin regulating high, I misunderstood your original question regarding this. Let me discuss with my team and I will get back to you soon.

Thanks,

Dino Maslic

Hi Dinesh,

Could you also send me your schematic and layout of the PCB?

Thanks,

Dino

Hi Dino, 

Please find the VR circuit schematic and layout below. 

This circuit is based on Webench Circuit design report. 

Hi Dinesh,

Can you send me all layers of your PCB? Need to see all of them to get a good understanding of your layout/routing.

Thanks,

Dino Maslic

Hi Dino,

I have attached PCB layout with all relevant layers in VR region.

Please let me know if you need any further details.

Thanks, Dinesh

Hi Dinesh,

Thanks for sending the PCB layout over. Question for you, does the FB pin voltage stay the same when the device is at no load vs fully loaded?

Thanks,

Dino Maslic

Hi Dino,

We see FB at same voltage level under nominal load, at present it could be anywhere between 1 to 2 A. 

I have not tested full load condition yet. 

Thanks, Dinesh

Dinesh,

Understood, going to talk to some members on my team about this and will get back to you as soon as I can.

Thanks,

Dino Maslic

Hi Dino,

Do you have any suggestions on the issue? 

Thanks, Dinesh 

Hi Dinesh,

Couple questions for you regarding the device:

-Can you send a scope capture taking device from 0 to full load? Showing SW node and inductor current if possible

-Where are you taking measurement of FB at? At the pin? Also  where are you probing your ground reference?

Thanks,

Dino Maslic

Hi Dino,

I measured FB voltage at pin using multimeter and reference ground was nearby quite digital Ground. 

I will try to capture low load to Full load waveforms (FB pin, VR output voltage, Electronic load current, SW pin voltage) in scope by connecting VR to an electronic Load. I will post the result once I have them.

But I am not sure if I can capture SW node and inductor current measurements. Are you referring SW node current as VR pin#19 (SW) current? 

Do you have any suggestion to measure SW pin and inductor current parameters without the need of removing the VR or inductor? 

Thanks, Dinesh

Hi Dinesh,

Sounds good.

In regards to probing SW, this is pin 19. yYou can just probe the terminal of the BST-SW capacitor to get a voltage reading of SW, no need for current. 

To get inductor current you would have to lift the inductor up unfortunately, let's stick to just the SW node voltage for now, if needed I may ask again for inductor current to get better idea of what is going on.

Thanks,

Dino Maslic

Hi Dino,

Below are the waveforms for reference.

Channel 1 - External Electronic Load Sink current

Channel 2 - FB pin voltage (pin# 5)

Channel 3 - VR output voltage (expected 3.3V)

Channel 4 - VR SW pin# 19

Apart from Electronic Load sink current, PCB itself may consume anything between 0A to 2A, I do not have accurate reading on this. 

Waveform also has few measurements for reference.

Thanks, Dinesh

Electronic Load Sink - 0A

Electronic Load Sink - 1A

Electronic Load Sink - 2A

Electronic Load Sink - 3A

Electronic Load Sink - 4A

Electronic Load Sink - 5A

Dinesh,

Thanks for sending the waveforms over, I will convene with my team and get back to you soon.

Thanks,

Dino Maslic

Hi Dino,

Do you have any feedback on the reported issue?

Thanks, Dinesh

HI Dinesh,

Have you tried any other units on the board?

Thanks,

Dino

Hi Dino,

I managed to check the VR output voltage on two of other sample boards and it is coming around 3.31V, which is as expected.!

LM25145RGYR VR IC on first module seem to be having issue, do you agree?

Thanks, Dinesh

Hi Dinesh,

If you put this device onto the LM25145 EVM, do you still see an issue with regulation?

Thanks,

Dino

Hi Dino, 

Sorry I cannot try that as I do not have EVM.

Thanks, Dinesh

Hey Dinesh,

Can you swap the part that has the higher FB voltage on one of the boards that has a working part? Just trying to rule out anything with the boards.

Thanks,

Dino Maslic

Hi Dino,

I do not see the possibility of replacing/swapping the VR part. VR is working fine on other 4 sample boards but not so accurate only on one board. 

For the time being, I will go with assumption of VR IC specific issue on one board. 

Thanks, Dinesh